Automatic system of control



Feb. 3, 1925. 1,524,930

R. V. L. HARTLEY AUTOMATIC SYSTEM OF CONTROL Filed Dec. 17, 1921 2 Sheets-Sheet 1 by A 212 Feb. 3, 1925. 4 1,524,930

R. v. L. HARTLEY AUTOMATIC SYSTEM OF CONTROL Filed Dec. 1'7, 1921 2 Sheots-$heet 2 $56: if T AMPLIFIER '5 A) T619 L/eg A P 0 2 v aaga/ [@166 i- LE 74 Q: 6'4 g Y I 3 I I;

Patented Feb. 3, 1925.

v UNITED STATES 1,524,930 PATENT OFFICE.

RALPH V. L. HARTLEY, OF EAST ORANGE, NEW JERSEY, ASSIGNOR TO WESTERN ELECTRIC COMPANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK.

AUTOMATIC SYSTEM OIE CONTROL.

Application filed December 17, 1921. Serial Nat 22,969.

To all whom it my concern:

Be it known that I, RALPH V. L. HARTLEY, a citizen of the United States, residing at East Orange, in the county of Essex, State of New Jersey, have invented certain new and useful Improvements in Automatic Systems of Control, of which the following is auxiliary to the transmission system itself.

A particular feature is the provision for the operation under control of the'signaling current of gain adjusting apparatus. This apparatus may advantageously take the form of, or include apparatus of. familiar type suitable for producing variations in the transmission characteristics of the signal transmission circuit. The specific gain adjusting apparatus illustrated and described in the present application is of the type arranged for varying the degree to which the received or repeated signalsare amplified.

In accordance with a further feature of the invention, in a multiplex system the adjustmen t of the gain for certain message currents may be effected under control of the same or of different message currents transmitted over the same system or circuit.

A. further feature of the invention comprises such an arrangement of the gain adjusting apparatus that this apparatus is opcrative only during the times in which signals or signaling current is actually being transmitted, the apparatus being disabled when the signals or signaling currents upon which it depends cease.

Automatic gain adjusting or gain indicating systems have been suggested heretofore in which the gain adjusting or indicating devices were actuated by energy other than the signaling energy. These have required the provision of one or more extra circuits or in the case of multiplex carrier wave systems, the use of a special channel or of some wave frequency or some current other than that actually serving to transmit the signals.

The invention by utilizing the signaling energy to accomplish both the signaling and gain adjusting functions avoids the use of extra apparatus heretofore thought necessary, and moreover, may utilize currents or frequencies most liable to changes affecting the transmission characteristics of a system, to compensate for variations in those characteristics According to the embodiment of the invention to be more fully described hereinafter, a relay controlled over the line connects at one of its contacts a galvanometer into and out of a local circuit containingma ballast source of E. M. F. and a resistance across which a drop of potential occurs also controlled over the line. When this drop of potential equals the ballest E. M. F. the galvanometer receives no current, but as this drop of potential varies due to changes in the line'transmission characteristics, the galvanometer is actuated and is caused to control automatically-acting apparatus which introduces more or less gain into an amplifier associated with the line.

Certain features and the general nature and objects of the invention have been briefly indicated above. These and other features and objects will be more clearly understood from the following detailed description of particular systems embodying the invention and illustrated in the accompanying drawmgs.

Fig. l is a schematic representation of a repeating station on a line which is preferably a multiplex carrier current l ne; Fig. 2, shows a terminal station which may belong to the line shown in Fig. 1; and Fig. 3, shows a repeater adapted for use in composite telephone and telegraph systems.

Referring first. to Fig. 1, the two repeaters RE and RW are shown arranged to provide a two-way repeating circuit between the line" sections LW, LE. These line sections are shown provided with balanced repeating coils H H respectively, and with line balpermitting this current to find its way into the receiving branches 14 :or 10 as is well known in the art. The tworepeating circuits RE and RW may be entirely similar,

and each comprises a pair of amplifying 1 334,681, filed October 31, 1919, may be consulted, this particular type of amplifying circuit not forming any part of the present invention, but having been chosen for illustration because of its particularadaptability for use in multiplex transmission circuits. I

According to the invention this two-way amplifying circuit, which of itself is well known in the art, is provided with automatic adjusting apparatus which will now be more fully described. This adjusting apparatus comprises preferably means for varying the potential that is impressed upon the amplifyingdevices in 'order to compensate for variations in the transmission characteristics of the line. In the input circuits of the repeaters RW and RE variable rheostats 18, 19, 21 and 22vare shown which may be controlled fromf'acommon shaft indicated at 20. The '"sl1afta'20 can be rotated in either direction by thesteppin'g mechanism generally indicated at 25 the arrangement being such that. the she. .is rotated in a clockwise direction by the spur wheel and I in a counter-clockwise direction by the spur wheel 51. These spur wheels are arranged to be actuated selectively by the magnets 47, 48, 52and 53 under control of a galvanometer'40 which is in turn controlled by the detector tube 36 and relay 37 connected in the output of, the detector. The detector 36 responds to current transmitted over the line and through the repeater RE.

The circuit having been generally described, its specific nature will be more clearly understood from a detailed description of its operation. Current received over the line section LW passes into the circuit 14, through the transformer 15 and potcntiometer resistances 21 and 22, and produces a potential on the ids of the discharge tubes of repeater R Corresponding output current variations pass through the arge condensers 29 and 30, through the coupling 16 into the out oing circuit 17 and from thence by way 0 the balanced repeating coil H into the line LE. In a similar manner current variations received over the line LE are transmitted through the circuit 10, transformer 11, repeater ltliV, coupling 12, outgoing circuit 13 and balanced repeating coil I-I intothe line'section LIV.

The currents in the outgoing circuit 17 produce a difference of potential across the impedance 31 and as-a result current of a definite frequency is transmitted through the path made up of the transformer 32, tuned circuit 33, transformer 34 and tuned circuit 35. This path is preferably arranged to transmit current of a particular one of several carrier channels which may be associated with the line L'W, LE, and these -carrier channels may be arrangedfor transmitting telephone conversations, telegraph messages or other signals. Current so selected from the outgoing circuit 17 is impressed on the circuit of the detector 36, which is preferably of the well known vacuum tube it type. In the output of detector 36, a relay 37 and a resistance 38 are included in series. A small condenser 44 may advantageously be connected in shunt of these elements to serve as a by-pass for the high frequency current components. Due to this wellknown detector circuit arrangement the relay- 37 maybe adjusted to energize when current of the frequency to which circuits 33 and 35 are selective is being transmitted over the line, but to deenergize when current of this particular frequency is cut off.

Assuming that the lines LVV and LE are arranged to transmit a number of high frequencies controlled in accordance with telegraphic code signals and that one such high frequency is selected and impressed on detector 36, the relay 37 will respond to the telegraphic impulses. Or, if the line LW, LE is arranged as'a multiplex carrier telephone circuit in which the unmodulatcd carrier frequency component of one or more channels passes over the line,the relay 36 may be arranged to respond to the carrier component of a definite one of these chan nels. In either case it is evident that the relay 37 will be energized at certain times and deenergized at other times. For example, relay 37 may be deenergized between the telegraphic code impulses or in the case of a telephone line during the intervals in which the carrier current is cut off from the line for some purpose, such as in transmitting ringing or other signals.-

.VVit h the relay 37 energized a circuit is closed including battery 39, which may be termed a ballast source of potential, coil 41 of the galvanometer 40 and a portion of resistance 38 determined by the setting of the movable contact 43. If the potential of battery 39 is equal and opposite to the drop of potentiahacross the portion of resistance 38 that is included in the circuit of the.

galvanometer coil when relay 37 is energized, the galvanometer will receive no current and its contact will occupy a midposi the galvanometer tongue in'its midposltion when no current is present in the coil of the instrument.

The drop of otential across resistance 38 depends upon 1; e strengthof signals in the outgoing circuit 17, which in turn depends upon the strength of signals received over the line LW and the transmitting charac= teristics of the repeater circuit RE as a whole. If the transmission characteristics of the line LW undergo a change due for example to wet weather or to a change in temperature or to any other conditions, the strength of signals received at the repeating point will tend to vary. Assuming that a chan e occurs in the transmission characteristics of the l ne LW in such direction as to weaken the slgnals received at the repeating point shown, the signals in the outgoing circuit will also be weaker and, as stated, the difference of potential set up across resistance 38 by the signal current will be smaller. It follows then that with relay 37 energized, and with the drop of potential that is applied to the galvan'ometer circuit from. resistance 38 when relay 37 is energized less than the potential corresponding to the balanced condition, thegalvanometer will be deflected. If it is assumed that the deflection is in such direction as to close contact 46, a clrcuit will then exist. from battery through the winding of relay 42' and its back contact, contact 46, relay-52 and relay 53 in series. Relay 52in energizing lifts its armature out. of engagement with spur wheel 51. Relay '53 in energizing. causes spur wheel to advance one step in a clockwise direction. Relay 42 in energizing an instant after the other relays have operated,-breaks' the energizing c ircuit previously traced through the back contact of this relay and relays '42, 52 and 53 deenergize; When the spur wheel 50 was advanced one step as described all four arms of the rheostats 18, 19, 21 and 22 were advanced a step in the clockwise directionunder control of shaft 20. The movement of the rheostat arms in this direction shifts the'points at which the respective grids of the amplifier tubes are connected to the potentiometer resistance 18, 19, 21 and :22 in such a way as to cause a greater pro portion of the potential generated by incoming signals across these potentiometers to be impressed upon the respective grids.

The amplifiul output currents will be correspondingly strengthened and the potential difference set-up across resistance 38 by a particular one of these signaling currents will become greater. If, however, the battery 39 still overbalances the potential derived in the galvanometer circuit from resistance 38, the galvanometer will keep contact 46 closed and when relay 42 deenergizes, the circuit previously traced will be again closed through relays 52 and 53 causing shaft 20 to take another step in the clockwise direction. This stepping of the 'shaft 20 will continue as long as contact-46 remainsclosed. When, however, the potentiometers 21 and 22 have been advanced a suflicient distance so that the increased p'otential applied to the grids causes the potential difference derived by the galvanometer coil from resistance 38 again to, equal the potential of battery 39, the galvanometer will restore to its midposition opening the contact 46. y

If, when a balanced condition of the galvanometer circuit exists, the line transmission characteristics should improve and the received. signals become strengthened the potential difference derived in the galvans ometer circuit from resistance 38"" when relay 37 is energized tends to overbalance the potential of battery 39 causing the galvanometer to deflect to close contarJ: A5. In this case, a circuit is closed from battery, winding of relay 42, armature and back contact of this relay, contact 45, and relays 47 and 48 in series. Relay 47 in energizing withdraws its armature. from engagement with spur wheel 50. Relay 48 in energizing attracts its armature to advance spur wheel 51 a step in the counter-clockwise direction, and an instant later relay 42 energizes, opening the previously traced energizing circuit at its back contact.' This steppingof spur wheel 51 and with it the shaft 20 and arms of rheostats 18 1-9, 21 and 22decreases thepotential applied to the grids of the repeating tubes by incoming signals. This causes the potential derived from the resistance 38 by the galvanometer circuit to fall off and tend to restore the galvanometer to normal. After asuflicientnumber of similar steps, the galvanometer is restored opening contact 45 and allowing the correcting mechanism 25 to return to rest.

If the im ulses to which relay 37 respond are of rapid occurence, it may be of ad-,

after several vibrations of the armature of Y relay 37 and remains closed during the continued vibration of the armature provided an unbalanced condition exists between the potential of battery 39 and the potential difference derived from resistance 38 when the galvanometer circuit is closed. Y

In practice the potentiometers 18, 19, 21 and 22 may, of course, be made to have a delioate adjustment so that a single step of the shaft 20 cil'ects only a slight change in the adjustment of the repeater circuits.

A correcting mechanism 25 controlling the input potentiometers of both repeaters RVV and RE is shown arranged to be actuated by currents received over the line section LW. In cases where the transmission characteristics of the line sections L and LE are similar and are subject to similar variations, this arrangement of apparatus will suffice. Under other conditions, the potentiometers 18 and 19 may be controlled by another set'of apparatus entirely similar to that generically indicated as 25, through the medium of a selective path and a detector and galvanometer, preferably associated with 'the outgoing circuit- 13 in the same way as the apparatus shown is associated with circuit 17.

Referring now to Fig. 2, a portion of the terminal circuits of a multiplex carrier current telegraphic system are shown. A line LE arranged for multiplex transmission terminates in the balanced repeater coil H and the balancing line network N,. The transmitting branch 58 of this repeating coil 'is' shown connected through the selective circuits 56 and 57 with carrier transmitting apparatus adapted to be controlled from a receiving Morse telegraphic circuit RT. Current impulses in the circuit RT cause relay 55 to vibrate its armature. This may be arranged to control in any suitable manner the oscillator-amplifier circuit diagrammatioally indicated, for producing code impulses of a high frequency wave for transmission to the line LE by way of the common transmittting branch 58. It is to be understood that other carrier Wave transmitting branches employing other wave frequencies may be similarly connected to the common branch 58.

The receiving branch59 of the repeating coil is shown provided with bridged circuits 60, 62 and 64, which together with secondary tuned circuits 61, 63, etc., form selective paths for the currents of the various high frequencies to be separately received fromv the line. These paths may lead to similar translating circuits. The path 60, 61 is shown leading to an amplifier A and-a detector D arranged in tandem for controlling a relay 65, which in turn repeats code impulses received over the line LE into the telegraphic transmitting line TL. The detector D may be entirely similar to detector 36 of Fig. 1 and its output circuit may contain in addition to relay 65 :a resistance llattcry 39 and galvanor'neter 46 are also provided in a manner entirely analogous to that described in connection with Fig. 1 for controlling the correcting mechanism 25. The spur wheels 50 and 51 of this correcting mechanism are arranged to rotate in one or the other direction a shaft indicated at 70, which controls in common" the settings of separate potentiometer switches as 68, 69, etc., one for each of the receiving channels. These potentiometers control the potential impressed upon the input circuit of the amplifier A of each channel by the currents selectively received by each respective channel.

The operation of the circuit 'of Fig. 2 will be understood from the description in connection with Fig. 1. With the galvanometer 40 inits balanced or midposition, the potentiometers 68 and 69 maintain a definite setting. As the transmission characteristics of the line LE undergo a change, however, the potential difference across the resistance '38 in the output of detector D also varies and as a result the balance between the constant potential of battery 39 and the potential derived in the galvanometer circuit from resistance 38 no longer exists. This causes the galvanometer coil to deflect to close a circuit over contact 45 or 46. As previously described, so long as either contact remains closed, thestepping relay 42 produces impulses in the circuit of one or another of the stepping relays48 or 53 and the corresponding releasing relay 47 or 52 causing the shaft 70 to rotate in one or the other direction. This action changes the setting of the respective potentiometers 68, 69, etc., in such direction as to tend to maintain the strength of the signals received at 65 at a constant value.

In Fig. 3, the invention is shown applied to an ordinary telephone repeater employed on a line which is composited for transm-is sion of telegraphic or other code impulses. The line is shown as divided in the two sections L L between which the respective one-Way repeaters E and VV lare arranged to form a two-way repeating circuit for voice currents. Line L has a bridged impedance forming a direct current path from the upper side of the line through the choke coil 78, conductor 77, key 102, line battery 101, resistance 38, relay 95, conductor 76, choke coil 79, and the lower side of the line. This series circuit may becontrolled in any suitable manner at a distant point or locally by the key 102 to cause actuation of relay 95. Impedance elements 80 and 81 are connected across the conductors 76 and 7 T for preventing transmission of voice currents to the circuit of relay 95, and the chokes 78 and 79 are provided with the usual condensers and grounded conductors which togetherwith the chokes 78 and 79 may be suitably arranged to maintain a proper balance of the circuit and to prevent the voice currents from traversing the bridged circuit. Condensers 82 and inductance 84 further serve to separate voice currents for ex elusive transmission to the telephone repeaters. The repeater circuit E is assoclated on its input side by means of couplin s 85 and 86 in bridge of the line L while it is associated in series relation with the line L through its outgoing couplings 87 and 88. Elements 78', 79, 82, 83' and 84 are supplied in duplicate to the corresponding elements on the line side of the repeating coil and network this the electrical equivalent of the line circuit L so that the bridged coupling 85 is maintained neutral with respect .to currents impressed on the line circuit L by means of coil 92. Linev L may terminate at the repeater in circuits identi' cal with those of L but for the sake of simplicity in the drawing, the various elements of the terminal circuit have not been shown. The repeater W is associated on its input side with the line L by means of the bridged coupling 89 and the transformer 90, and its output side is associated through couplings 91 and 92 with the line circuit L Adjustable potentiometers 98 and 94 are shown controlled by a shaft indicated at 100 arranged to be actuated by correcting mechanism 25.

Due to the well-known action of a composite system of the general character indicated in this figure, telegraphic impulses and voice currents may be transmitted over the line L without disturbing each other. At the repeating station shown, the provision of the impedance elements as described enables the separation of telegraphic impulses into the circuit of relay 95 and the telephonic frequencies into the circuit to the right of transformer 83 in the drawing. Voice currents received over the line L are repeated into the line L by the repeater E, and similarly voice currents are repeated through repeater W from the line L to line L Curient impulses on the line L cause relay 95 to attract and release its armatures, the upper armature operating the sounder S -in the local circuit indicated. The lower armature when attracted closes a circuit including a portion of the resistance 38 determined by the setting of the contact 43, lower armature and contact of relay 95, coil of galvanometer 40, resistance 103, and in shunt to resistance 103, adjustable resistance 99 and battery 97. The combination of battery 97 and resistances 103 and 99 replaces in a general way battery 39 in the circuits heretofore considered, the battery 97 serving to produce a potential difference across resistance 103 of such sign and magnitude as normally to balance out the potential diflerence the included in the series line circuit ofacross the portion of resistance 38 that'l's included in the circuit of the galvanometer when relay 95 is energized.

Assuming that the transmission characteristics of the line L remain constant, also that the line batteries remain constant, the potential diiference across resistance 38 when the line circuit is closed will also reto line battery 101 (and any other batterifis main constant. If this potential diflz'erence balances the potential difierence between the terminals of the resistance 103, the galvanometer receives no current and remains in its neutral position. A common cause of variation in the transmission characteristics of a telephone line is a variation in temperature producing an increase or decrease in the resistance of the line conductors. When for this or any other reason a change in the resistance of the line circuit occurs altering the drop of potential on the line conductors, the potential drop across resistance 38 also varies. The balanced condition existing 'between the two potential sources in the galvanometer circuit, consisting of both of the potential drops across a portion of resistance 38 and across resistance .103 is upset and the galvanometer 4O deflects to close one or the other of its contacts 45 or 46. When either of these contacts is closed, the stepping mechanism 25 is brought into action 1n a manner described in connection with the previous figures of the drawing and as a result the setting of the adjustable potentiometers 98, 94 and 99 is varied. The potentiome ters 98 and 94 arein this way adjusted in a direction to increase or decrease the voltage impressed on the amplifier elements and direction as to tend to restore the balaneeexisting between the two potential drops across the resistance 103 and that portion of resistance 38 included in the galvanometer circuit, respectively. An. empirical relation exists between the potentiometers 98 and 94 and the potentiometer 99 so that this balance between the opposed E. M. Fs in the galvanometer circuit is restored just at the time when the proper adjustment has been made of potentiometers 98 and 94 to compensate completely for the variation that is supposed to. have occurred in the line resistance. It is evident, then, that contact 45 or 46 will remain closed under control of relay 95 and maintain the stepping mechanism 25 in operation until the change in the gain of the repeaters W and E corresponds in direction and amount to the change in line impedance in such a way as to compensate for the change in line impedance, and that when this adjustment of the repeaters has been made potentiometer 99 will have introduced more or less resistance in series with battery 97 and resistance 103 producing a new condition of balance in the galvanomv eter circuit and restoring the galvanometer to its midposition in which both contacts 45 and 46am opened.-- A further change in the line impedance will, in a similar manner, upsetthis condition of balance causing the stopping mechanism 25 to be brought into could be supplied for separately adjusting the repeater W and that it could b' con-- [trolled by current impulses transmitted over received si the line section L if desired. V

The systems that have been illustrated and described should be considered merely as typical and not as limiting the invention, the scope of which is defined in the appended clalm's.

What is claimed is:

1. In a signaling system in which the strength of the received signals incoming over a given line is subject to change from time to time, an amplifier for the received signals, and means operable only during the receipt of signaling current and dependent for its operation upon the strength of received signals forautomatically varying the amplification by said amplifier.

2. In a receiving system, a cathode-grid anode discharge tube for receivin signals, an adjustable device for varying t e poten-. tial impressed on the grid by a received signal of given strength, and means actuated by the received signaling currentfor automatically controlling said device in accordance with the strength of the signals received, said means being dependent for its operation upon the receipt of signaling current.

3. In a signaling system, means to trans mit and'receive signals, an amplifier for the received signals, an automatically operating control circuit for said amplifier, and means dependent for any operation on a change in the strength of sa1d received signals to initiate the operation of said control circuit.

4. In a signaling system, means to transmit signals, an amplifier for said signals, a step-by-step adjustable potentiometer for said amplifier, a signal indicator controlled by said si als and automatic control means actuated said indicator for variably adjustin said potentiometer in accordance with t e strength of the received signals.

5. Ina signaling system, a line, means for receiving signals transmitted thereovery an amplifier associated with said line, and

means controlled by received signals for variably adjusting said amplifier ;1n accordance with varigtions in-the strength of'the a Y 76,, In a signaling system, a line for transmittin signals of varying strength, an ampli or associated with said line, and

an amplifier associated with said line for amplifying signals transmitted with variable amplitude tliereover, means for receiving the amplified signals, and means controlled'by the amplified signals for varying the amplification of signals in an inverse manner to the variations in the llne attenua tion.

8. In a signaling system, a line, the attenua'tion of which is subject to variations, 7

means to transmit signals'over said line, an auxiliary circuit associated with said line, and means controlled by said signals for varying the signal attenuating characteristics of said auxiliary circuit in a manner nation of said line.

9. In a signaling system, a line, the attenuation of which is subject to variations, means to transmit signals over said line, an auxiliary circuit associated with said line oomplemental to the variations in the attenfor also transmitting signals that are transmitted over said line, said auxiliary circuit being independent of variations in the line attenuation, and means controlled by signals transmitted over the line for introducing variations into the transmission characteristics of said auxiliary circuit for maintaining the total attenuation of said line and auxiliary circuit together substantially constant.

10. In a signalin system,- a transmission circuit, an amplig'ing circuit I associated therewith, means to transmit signaling currentsof different character simultaneously over said system, and means acting automatically under control of signaling currents of one character for adjusting the characteristics of said amplifying circuit for currents of another character.

11. In a signaling system, a transmission circuit, an adjusting apparatus for said circuit having a movable element, means for utilizing said circuit to transmit simul taneously a plurality of currents of different character and to transmit selectively signals bl'y said currents, and means controlled by t e current of one character for giving signal indications and for controlling said apparatus.

12. In'a signaling system, means to transmit current and to control the current to indicate-signals, an amplifier for said signals,

13 In a. signaling system, a carrier sig.

naling channel, signaling apparatus individlet In a c'arnerwave signaling system,

means for. transmitting a carrier wave and for impressing signals thereon, an amplifier It for amplifying said Wave, and automatic .plifying means for variably adjusting said amplifier during the transmission of said wave, sa -id automatic means being dependent for its operation upon receipt of said Wave.

V 15. In a multiplex carrier current signaling system, a line, carrier current signaling channels superposed on said line, amplifying means arranged to amplify the currents transmitted by-a plurality of said channels, and I automatically operating adjusting means controlled over a single-one of said signaling channels "for adjusting said amphlgying means for said plurality of channe 16. Ina carrier wave signalingsystem, a plurality of carrier signaling channels employing distinctive carrier frequencies, ammeans for. said channels,means actuated by the amplified energy of one of said channels for acteristics of said amplifying means for'all of said channels. 1

17. In a carrier wave signaling system, means to transmit a carrier wave and to interrupt said wave to transmit signals, an amplifier for said wave, and apparatus responsive to the continuous and 'to the intermittent transmission of said wave for variably adjusting thetransmissioncharacteristics of said amplifier during-"the transmission.

' 18. In a signaling system, a relay responsive to received signaling current,- a resistance included in the path of received signaling current, and a galvanometer and circuit therefor, said relay being arranged upon receipt of signaling current to connect said resistance into the circuit of said galvanometer.

varying the transmission char- 19. In a signaling sy an amplifier, a relay responsive toreceive signaling current, a resistance included, in the path of received signaling current, a galvanometer and a circuit therefor including a ballast source of potential, said relay being arranged upon receipt of signaling current to connect said resistance in circuit with said galvanometer and said ballast source of potential, and means controlled by said galvanometer for modifying the action of said amplifier.

20.- In combination, means for transmitting energy from a distance, a balanced galvanometer circuit com rising a locally controlled ballast source 0 potential and a source of potential produced by signaling ener received from a distance, said sources being arranged in. opposition in said 1- vanometer circuit, and a relay also c'ontro ed by received signaling energy for cutting said last-mentioned source of potential into and'out of the circuit of said galvanometer.

21. In a signaling system, an'amplifier for received signals, a relay responsive to received current impulses, a resistance included in the path of received current impulses, a galvanometer and a ballast source of potentential, said relay being arranged upon its energization to close a circuit including-said resistance, said galvanometer and saidballast source of potential, said ballast source. being connected to oppose the drop of potential across said resistance due to received impulses, and means controlled by said gal .vanometer for changing the gain of said amplifier in such adirection as to tend to keep the drop of potential across said resistance substantially constant during the reception. of signaling current. 22. In a signal receivin system, a plurality of amplifiers in par el each arranged toamplify received signals, and means controlled by one of said amplifiers for altering the circuit of each of said amplifiers.

In witness whereof, I hereunto subscribe my name this 14th day of December A. D.,

RALPH v. L. HARTLEY. 

